Dynamic diode tester



Feb. 9, 1954 B. T. WILSON 2,668,943

DYNAMIC Drome TESTER Filed Feb. 16, 1951 www@ Patented Feb. 9, 1954 DYNAMIC DIODE TESTER Bernard T. Wilson, Los Angeles, Calif., assigner, by mesne assignments, to Computer Research Corporation of California, a corporation of Delaware Application February 16, 1951, Serial No. 211,241

This invention relates to crystal diode testing devices and more particularly to means for testing crystal diodes under dynamic operating conditions as met in computer circuits.

Crystal diodes or rectiers are very useful as components in diode networks for interconnecting flip-flops as required in binary digital computer systems. It has been found that all the logical operations needed for operating on the digital information set up in the computer can be performed by gating and mixing circuits comprised primarily of crystal diodes and resistors. Consequently as manyas a thousand or more of these diodes may be used in one computer.

Crystal diodes have heretofore been tested for the ratio of forward to back resistance. These tests are normally carried out by applying steady state D. C. voltages across the diodes causing them to conduct in both directions. The ratio of the resistance readings then provides a useful criterion as to the rectifying ability of the diode.

Extensive use of diodes in computer networks has now revealed that another characteristic of diodes is of extreme importance. This characteristic is actually of a transient or dynamic nature in that when a D. C. voltage is impressed across a diode, the forward resistance at the fir-st instance of the application of the D. C. voltage may not be the same as for the steady state condition.

Because of the manner of using these diodes, to form gates, for example, this characteristic prevents the trailing edge of the output pulse from the gate, when differentiated, from giving the desired signal.

Briefly stated, the present invention comprises a means for applying rst a symmetrical square wave voltage and then a steady state D. C. `voltage having the same peak value, across the diode to be tested. If the test diode has good transient characteristics a voltmeter across the diode would read `exactly one-half the steady state reading when energized by the square wave. Actually, switching means are provided for increasing the -sc'ale of the voltmeter by two when the steady D. C. is applied. Thus a consistent reading on -the voltmeter is indicative of a diode with good 'underl dynamic operating conditions.

6 Claims. (Cl. 324-57) This invention will be better understood from the following description considered in connection with the accompanying drawings and its scope is indicated by the appendedclaims.

Referring to the figures of the drawings:

Figure 1 is a circuit diagram of a diode gate circuit as used in digital computers revealing the nature of the dynamic characteristic of a good and bad diode on the gate output signal waveform.

Figure 2 is the circuit diagram of the present invention which provides means for testing the forward resistance of the diode under dynamic operating conditions.

Figures 3a, 3b, and 3c are graphs that are referred to in explaining the operation of the circuit shown in Fig. 2.

Referring first to Figure l, a typical diode gate circuit I0, as used in a digital computer, together With the waveforms shown, illustrates the particular operating characteristic of the crystal diode which is to be tested by the circuit comprising the present invention.

Here it is seen that the right and left inputs II and I2 respectively to the gate I0 are fed through the cathode equivalent ends of diodes I3 and I4 whose plate ends are joined to a common junction I5. This junction I5 is connected to a positive source B+ through a load resistor Il.

This gate circuit I0 is such that normally current flows from the positive source B+ through load resistor I'I; through each of the diodes I3 and I4 in parallel to the low potential of the inputs. Thus common junction I5 from which the output of the gate I0 is taken is normally at a relatively low potential level.

It should be noted that because of nonlinearities in the voltage vs. current characteristics of individual diodes, the current flow through each of the two diodes I3 and I 4 as shown in gate I0, may be considerably different. 1t is possible, for example, that substantially less current might flow through say the left diode I4 than the right diode I3 for the same voltage across the diodes.

In operation of gate I0, the potentials applied to the inputs I I and I2 can only have two values. Any time right input I I has a high potential such as shown by wave I9, a square wave clock pulse 20 applied on the left input I2 reduces the current flow through the load resistor I'I for the duration of the clock pulse, thus bringing common junction I5 during this period to its relatively high potential. Describing this event in greater detail, the step potential at` the. leading edge of the clock pulse suddenly reduces the flow of currentl through load resistor I1 so that the leading edge of the output wave 2| is relatively steep; however, the trailing edge of the clock pulse 20 again impresses a voltage across the left diode I4 so as to cause it to increase the currentilow there through inra forward direction. It is the sudden applicationl of an' increased voltage across the left diode I4 that produces the undesirable transient effect when the diode is bad. The initial resistance of al bad diode is large; thus an appreciable time is'" required for the trailing edge of. the output. pulse 2l from the gate l0 to drop-td the steady-'state' conducting value. An oscilloscope122 connectedi across the left diode i4 would show a wave pattern such as 23 on its screen when the .diode lil.

is functioning improperly.

As seen in Figure 1, this output square' wave pulse 2| from the gatehll) is differentiated by means of capacitorY 25A and resistor 25: and the positive leadingwave is then attenuated inv a series diode 21.. The negative trailing. waveiwhich passes through the series diode 21 is the output signal 2S which is applied tothe grid of triode3 for triggering off. the left tube' 30 of a flip-flop 29.

Flip-Hop 2) is a conventional two stable state circuit andl will not be' further described..

When the left diode I4 is functioning as required, the output pulse from the gate it has a negative pulse formas shown by the'solid output signal 28'. This signal is of theV required negative magnitude not only to cut off the* tube 3B but also to include a safety factor desired by the designer of the circuits for ageing of the tubes. Onv the other hand, if the' diode I4r has the undesirable dynamic characteristic, the output gives a negative pulse having a form' as shownk by the dotted output signal 28. Thisla-tter'signaldoes not have the magnitude requiredfor triggering the tube 3i?.

Thus it can be' seen that the reliability and life of these computing circuits are greatly improved by diodes having proper'dynamic characteristics'.

Referring next to Figure 2, a preferred circuit diagram of the present-invention' for testingthis dynamic flz'r-wardly characteristic of a diode is shown.

Here square wave generator 32, feedingk through a cathode follower-'33, emits a symmetrical square wave train (similar to that shownfin'. Figure 3c) into one terminal? of a first two-way switoh. lThis square wave' has for example a- 50 v'. peak voltage value; A 50 v. D. C. steady state' voltage having the same base potential as the square wave is suppliedbysource 36. is applied to the other terminal of the switch The output of switch 35 contains an adjustable resistor`38 in series withanf ammeter 39. A diode 40 which is to be tested is positioned between the ammeter 39 andY ground.- This diode 4G is orientated so that the applied potential causes the diode 4i) toy conduct in a forward direction. The positiveA terminal of the diode 4i) is connected by a: linerli'l to a second two-way switch 2 coupled to be manually operated simultaneously with they first switch 375'. When nrst switch 35 connects the square" wave across the circuit, the second switch 4'2 is connected to provider agiven scale range reading on the vacuum. tube voltmeter'4'4; and, when the first switch 35 connects the D. C. voltage across the circuit, the second switch 42 is connected' through scale resistor 43 so as to increasev the' scale range of the vacuum tube voltmeterf 44= byt two.

In'- operation,. Withl thafirstI switch vt'r inL the Thisrlatter voltage position as shown in the drawing, connected to the square wave generator 32, the current adjusting variable resistor 38 is set so that the current meter 39 reads some safe value, say 25 ma. Since the square wave is symmetrical, and the ammeter 39 is inherently an averaging device, this reading will correspondv to'. a peak forward current through the test dioide 40 of5`0 ma. At this peak current, the test diode 40 will have a voltage across it depending on its forward resistance at this current. Assuming the test diode 40 under consideration has a good dynamic characteristic, thisvoltage will' be a square wave, as shown in Figurer 3a.. This' square wave voltage is averaged by the Vacuum tube voltrneter 44, so that it reads one-half off' the peak voltage developed across the test-diode 40;

If the rst switch 35 is now changed to connectL the D. C. source V35 across the circuit, the voltage across the diode 4t will be as shown in Figure 3c. The-actual current andvoltagevalues willnow berexacty twice the average'squarewave` value; However,. simultaneously.. the scale range of the voltrneter 44 has been-increased by two so that the reading thereonl should be the same..

Now suppose the diode 40 -is faulty'fin that it has a high forward resistance'V during the: first instant of forward conduction'. This will result in the wave across thediode 4i)s when impressed by the square'wave, to havethe form as shown in Figure 3b; This will cause the voltage: reading on the' voltnieter dit. upon application:y of the square wave voltage to the' circuit, to kbe actually greater than half the reading thereon upon--application of the1 steady` D. Ct voltage; butl since the vacuum tube voltmeter 44 is any averaging device and thescale'has been halved, any `deviation between readingsV of thev vacuumA tube voltineter 44 when the test` diode 40 is energized'. by each source is a measure' of the area 4510i the wave# forms of Figuref. This` area corresponds/to. the dynamic resistance variation ofthe test.- diode 40 and by proper calibration'of` the' vacuum: tube voltmeter 44, diodes having undesirable: dynamic characteristicsr cany be easily detected..v

While the circuit as shown andl described. here'- in is admirably adapted to'fullllthe: features? of advantage previously enumerated asy desirable; itis to be understood that the'inven'tion is-notto be limited to the specific features shown but that the means andA construction herein discloseduare susceptible of modification in form, proportion andV arrangement of parts'without departing; from the principle. involved or sacrificing any of its advantages, and the invention is; therefore claimed* in embodiments of various formsallcoming' within the scope of they claimsA which follow;

What is claimed:

l'. A dynamic crystalv diode testing. circuit comprisingv a source of DLC. potential, asource of symmetrical square wave potential having the same peak and base values as said D: C.- potential, a circuit including. a resistance in serieswith said crystal diode, a1two-position switch -for connecting said sources one ata timeA across. said circuit, andv a' voltrneter adapted for indicating average measurements connected across said crystal diode, whereby theaverage potential value across said crystal diode during application of said square wave is one-half the potential value during application of said D. C. source when said crystal diode has a favorable dynamic response.

2. A. circuit for the dynamic* testing ofl rectiers comprisingzi means for providing allv C. potential; means for` providing a squarefswalve potential having the same limiting values as said D. C. potential; connector means for establishing a connection to said rectifier; a circuit including a resistor in series with said connector means; switching means for selectively connecting either of said potential means to said circuit; and means for measuring the potential developed across said rectifier on application of each of said potentials to said circuit.

3. In a testing device for measuring the forward dynamic response of a rectifier: means for providing a direct current; means for providing a current whose voltage has a square wave form with the same peak and base values as said direct current; conductor means for establishing a connection to said rectifier; a resistor in series with said conductor means; a first switch for selectively connecting said current-providing means to said resistor and conductor means to cause current to flow through said rectifier in a forward direction; a voltmeter connected across said rectier; and a second switch interconnected with said rst switch and operable to increase the scale range of said voltmeter when said direct current is applied to said rectifier, whereby the reading on said voltmeter remains substantially constant when said reotier has a favorable dynamic characteristic.

4. A circuit for measuring the forward dynamic response of a rectifier on application of a step voltage, comprising: means for providing a voltage having a cyclic waveform with a steep wavefront from a low to high voltage level; means for providing a substantially constant voltage whose value has a defined relationship with respect to the mentioned voltage providing means; conductor means for establishing a connection to said rectifier; a series circuit including a resistor and said conductor means; switching means for selectively connecting said voltage providing means to said circuit so as to cause current to flow through said rectifier in a forward direction; and means for measuring the voltages developed across said rectifier while each of said voltage providing means is connected to said circuit.

5. A circuit for measuring the forward dynamic response of a rectifier on application of a voltage having a steep wavefront, comprising: a first power means providing a voltage having a cyclic waveform with a steep wavefront from a low to high voltage level; a second power means providing a D. C. voltage whose magnitude has a defined relationship with respect to the average value of the voltage of said first power means; conductor means for establishing a connection to said rectiiier; a series circuit including a resistor and said conductor means; switching means for selectively connecting said first and second power means to said circuit so as to cause current to now through said rectifier in a forward direction; and a voltage metering system adapted to measure the average voltage across said rectifier and to indicate the departure from the defined relationship of said voltages, caused by a variation in the forward dynamic resistance of said rectifier in response to the voltage from said first power means.

6. A circuit for measuring the variation in forward resistance of a rectier on application of a voltage having a steep wavefront, comprising: a first source of power providing a voltage having a recurring waveform with a steep wavefront from a low to high voltage level; a second source of power providing a D. C. voltage whose value has a deiined relationship with respect to the average value of the voltage of said first source of power; conductor means for establishing a connection to said rectifier; a series circuit including a resistor and said conductor means; a rst switching means for successively applying said first and second voltage sources to said series circuit for causing current to flow through said rectifier in a forward direction; a voltmeter adapted to measure the average voltage across said rectier; and a second switching means interconnected with said first switching means and operable to increase the scale range of said voltmeter in accordance with said dened relationship. whereby the reading on said voltmeter remains substantially constant on operation of said switching means when said rectifier has a favorable dynamic characteristic.

BERNARD T. WILSON.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,933,274 Ludwig Oct. 31, 1933 2,408,858 Keizer Oct. 8, 1946 2,409,419 Christaldi Oct. 15, 1946 2,459,849 Stateman Jan. 25, 1949 2,517,977 Cole et al Aug. 8, 1950 2,574,682 Ancona, Jr Nov. 13, 1951 2,585,353 Strum Feb. 12, 1952 OTHER REFERENCES Electronics, Sept. 1944, pages 13S-140, 336, 338. 

